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Rainwater harvesting, an ancient technology, is gaining interest in low impact development to reduce urban runoff and treated water use. However, a major concern for many is the water quality of the harvest and particularly the first flush. This research is testing the hypothesis that a more site-specific first flush volume can be quantified in rainwater harvesting based on the roofing material, roof orientation, and geographical location. This paper focuses on the research methodology, as initial data is expected to be collected during July 2011. Data collection is to take place in Oklahoma City and Stillwater, OK, from actual storm events and simulated rainfall events, respectively. Two commercial buildings representing metal and built-up roof material will be evaluated, along with a simulated asphalt-shingle roof structure at the OSU-OKC campus. In Stillwater, eighteen simulated structures have been constructed with new asphalt shingles, new Galvalume sheeting and 60 year-old clay tile roofing. The roofs are oriented either north-south or east-west in order to determine if roof orientation, in relation to the sun and prevailing wind direction, has a significant impact on rooftop runoff water quality. Roof runoff is to be characterized by continuous monitoring and discrete-sample analysis of specific conductance, turbidity, total suspended solids, polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), metals, bacteria, and nitrate in rooftop runoff throughout a storm event. Results from this study are expected to further knowledge on the first-flush phenomena, as there has yet to be a universal consensus on what exactly constitutes a first flush.